The present invention relates to an improved composite aluminum brazing product or sheet and an aluminum brazing material useful as a core alloy in such product. More particularly, the invention provides an AA 3000 series type alloy having improved corrosion resistance because of the intentional addition of increased amounts of titanium.
The use of clad composite aluminum brazing sheet to join aluminum alloy components is well known. In vacuum brazing the parts to be joined are assembled and heated in a vacuum to an elevated temperature, such as about 1100.degree. F. At that temperature a cladding alloy on one of the parts melts and fuses the parts to each other.
Brazing sheet with core alloys AA3000 ("AA" is the Aluminum Association, Inc., designation for wrought aluminum alloys) or AA3005 clad on at least one side with an Al-Si brazing alloy are commonly used in the manufacture of automobile heat exchangers. While AA3003 and AA3005 in numerous applications are generally considered to be corrosion resistant by themselves, it has been observed that when they are used as the core alloy in a brazed heat exchanger, the extreme heat from brazing and the proximity of silicon in the filler alloy cladding seriously degrade their corrosion resistance.
AA3005 (an Al-Mn-Mg alloy) has a considerably higher strength than AA3003 (an Al-Mn alloy) as a result of its magnesium content, but the brazeability and corrosion performance are partially sacrificed in favor of higher strength. Other means of increasing strength without this compromise have not been found heretofore. Brazed aluminum heat exchangers are frequently subject to failure as a result of corrosion attack from the exterior. Since the total thickness of brazing sheet composite materials is typically comprised of up to 95% core alloy, it can be appreciated that the life expectancy of the brazed aluminum heat exchanger is largely dependent upon the corrosion resistance of the core alloy.
Brazeability is required for the manufacture of heat exchangers in an efficient and economical manner. Corrosion resistance is necessary for heat exchangers because some environments in which they are used are highly corrosive. High strength is desired for reducing the material requirements of the heat exchanger, for enduring the stresses during manufacturing, and for containing the operating pressures in the completed heat exchanger.
There is a need, therefore, for an aluminum alloy for use as a core alloy in aluminum brazing sheet with high strength, improved resistance to corrosion attack and excellent brazeability.